import numpy as np
# arr = np.array([[1,2,3],[4,5,6],[7,8,9],[10,11,12]])
# print('4x3的整数数组: \n',arr)
# print('数组的ndim属性: \n',arr.ndim)
# print('数组的shape属性: \n',arr.shape)
# print('数组的size属性: \n',arr.size)
# print('数组的dtype属性: \n',arr.dtype)


arr = np.array([1,2,3,4,5,6,7,8,9])
print(arr[2])
print(arr[1:5])
print(arr[:8:2])
print(arr[::3])